The present invention relates to systems for transmitting information emitted by transmitter equipment over a short distance to one or more receivers, the transmitter equipment being located in a semihemispherical field above the receivers.
1. Field of the Invention
The invention relates more particularly to the transmission of confidential information under the best conditions of coverage, discretion and redundancy. In fact, such ideal transmission conditions are sometimes difficult to obtain when, in such a semi-hemispherical field, there are located parasite sources. These parasite sources may be very varied. A very bright background or the sun induces noise in the receiver equipment, which is harmful to the good quality and safety in reception of the message.
The transmission system of the invention applies in particular to links between an aircraft carrier and one or more aircraft on the flight deck or else to message exchanges between patrolling aircraft, one of the aircraft then carrying the cooperative source transmitting the useful message, the other aircraft having reception systems.
2. Description of the Prior Art
A unilateral transmission system comprising an optical transmitter with modulable beam placed on the transmitting equipment, means for modulating said beam from said information, an optical detector placed on each receiver equipment and means for demodulating the signal output from the receiver is already known. Thus, the patent FR-A-2 352 452 describes a system of this type. It recommended the use of incoherent radiation so as to guarantee practically absolute discretion since the law of detection is then 1/d.sup.2, d being the distance from the receiver equipment to the transmitter; it further recommended to illuminate a point of the field by two or more transmitters.
This latter system applied in particular to links between an aircraft carrier and an aircraft located on the flight deck of this carrier so as to ensure transmission of an alignment message. This system used a non coherent optical carrier in the near infrared, with a wavelength of about 0.9 micron. The transmission source properly speaking was a set of LEDs of type GaAs mounted in series and modulated for all or nothing. The purpose of the optical transmission system was to form, from the beam emitted by the diodes, a beam of .+-.1.degree. in elevation and 45.degree. or 90.degree. in relative bearing. The whole of the beam was slightly inclined downwards and was situated at the height of the rudder units of the aircraft. Coverage of the flight deck of the carrier was provided by a combination of several transmitters oriented judiciously with respect to each other. The receiver was formed of an optical head located on the rudder unit of the aircraft and formed of six silicon photodiodes bonded to a hexagonal support so as to provide, in the plane perpendicular to the axis of said support, a practically omnidirectional reception diagram. A thin CdTe layer, placed for example on the photodiodes, eliminated a major part of the solar spectrum at reception. Anti-glare treatment favoured reception by the photodiodes of signals at 0.93 micron. The whole of the reception device was further protected by a cylindrical quartz window. A horizontal "slatted shutter" system associated with the photodiodes enabled:
illumination of the photodiodes by the sun to be prevented when the relative bearing of the latter exceeded .+-.7.5.degree.,
illumination thereof by the surrounding landscape to be limited. Finally, the outputs of the diodes were placed in parallel, on the receiver side, through a coaxial cable at the input of a preamplifier. The optical head assembly of the receiver, called "candle" was substantially a cylinder projecting from the rudder unit of the aircraft over a height of 8.7 cm and a diameter of 4.7 cm.
This known system provided good confidentiality. However, it has today multiple drawbacks. A first drawback relates to the reception assembly or "candle" which forms a constraint from the aerodynamic point of view, more particularly for supersonic aircraft. In fact, this reception assembly is a cylinder projecting from the rudder unit of the aircraft.
Furthermore, the above reception system comprises an omnidirectional relative bearing detector which cannot avoid the presence of parasites when the field of view contains the sun or a very bright background.
In addition, this known system does not satisfy present day requirements of discretion since the beams emitted are almost horizontal. For these latter horizontal beams, it is the atmospheric absorption and the 1/d.sup.2 law which ensure a certain confidentiality by attenuating the transmitted illumination, where d is the distance to the transmitter. Finally, the horizontality of the beam transmitted by the transmitter equipment forms a drawback for coverage of the aircraft carrier because of possible occultation by other aircraft of different types. The fineness of the beam prevents its use by aircraft having receivers at different heights.